1.前言
Glide 是一个图片加载框架，可以在 Android 平台上以简单的方式来加载和展示图片。 通过Glide.with(this).load(url).into(imageView) 一句代码就可以搞定。虽然调用方式很简单，但是内部的实现却不简单。下面将根据这句代码展开梳理 Glide 如何实现加载网络图片。
2. Glide.with(context)
Glide 类的继承关系如下所示：
在 Glide 类中，根据传入的参数不同，总共重写了6个 with方法，这里将以比较常见的 Activity 为例子去追溯实现原理：
@NonNull
public static RequestManager with(@NonNull Context context) {
return getRetriever(context).get(context);
}
@NonNull
public static RequestManager with(@NonNull Activity activity) {
return getRetriever(activity).get(activity);
}
public static RequestManager with(@NonNull FragmentActivity activity) {
return getRetriever(activity).get(activity);
}
@NonNull
public static RequestManager with(@NonNull Fragment fragment) {
return getRetriever(fragment.getContext()).get(fragment);
}
@SuppressWarnings("deprecation")
@Deprecated
@NonNull
public static RequestManager with(@NonNull android.app.Fragment fragment) {
return getRetriever(fragment.getActivity()).get(fragment);
}
@NonNull
public static RequestManager with(@NonNull View view) {
return getRetriever(view.getContext()).get(view);
}在这几个方法中，最终的返回类型都是 RequestManager ，内部的调用都是 getRetriever().get()。
getRetriever 方法：
private static RequestManagerRetriever getRetriever(@Nullable Context context) {
// Context could be null for other reasons (ie the user passes in null), but in practice it will
// only occur due to errors with the Fragment lifecycle.
Preconditions.checkNotNull(
context,
"You cannot start a load on a not yet attached View or a Fragment where getActivity() "
+ "returns null (which usually occurs when getActivity() is called before the Fragment "
+ "is attached or after the Fragment is destroyed).");
return Glide.get(context).getRequestManagerRetriever();
}先判断传入的 context 是否为空，当不为空则执行 Glide.get(context).getRequestManagerRetriever()。
Glide 的 get(context) 方法：
/**
* Get the singleton.
*
* @return the singleton
*/
@NonNull
public static Glide get(@NonNull Context context) {
if (glide == null) {
GeneratedAppGlideModule annotationGeneratedModule =
getAnnotationGeneratedGlideModules(context.getApplicationContext());
synchronized (Glide.class) {
if (glide == null) {
checkAndInitializeGlide(context, annotationGeneratedModule);
}
}
}
return glide;
}很明显这是一个 Double Check 方式的单例模式获取 Glide 对象，调用了 checkAndInitializeGlide 方法：
private static void checkAndInitializeGlide(
@NonNull Context context, @Nullable GeneratedAppGlideModule generatedAppGlideModule) {
// In the thread running initGlide(), one or more classes may call Glide.get(context).
// Without this check, those calls could trigger infinite recursion.
if (isInitializing) { //如果正在初始化中，则抛出异常
throw new IllegalStateException(
"You cannot call Glide.get() in registerComponents(),"
+ " use the provided Glide instance instead");
}
isInitializing = true;//正在初始化中
initializeGlide(context, generatedAppGlideModule);
isInitializing = false;//初始化结束
}调用了 initializeGlide 方法：
@GuardedBy("Glide.class")
private static void initializeGlide(
@NonNull Context context, @Nullable GeneratedAppGlideModule generatedAppGlideModule) {
initializeGlide(context, new GlideBuilder(), generatedAppGlideModule);
}又调用了另外一个 initializeGlide 方法：
@GuardedBy("Glide.class")
@SuppressWarnings("deprecation")
private static void initializeGlide(
@NonNull Context context,
@NonNull GlideBuilder builder,
@Nullable GeneratedAppGlideModule annotationGeneratedModule) {
Context applicationContext = context.getApplicationContext();
...
//设置RequestManager的工厂类
RequestManagerRetriever.RequestManagerFactory factory =
annotationGeneratedModule != null
? annotationGeneratedModule.getRequestManagerFactory()
: null;
builder.setRequestManagerFactory(factory);
...
// 使用GlideBuilder根据配置信息构建Glide对象
Glide glide = builder.build(applicationContext);
...
//glide监听Application的组件生命周期
applicationContext.registerComponentCallbacks(glide);
Glide.glide = glide;
}通过 GlideBuilder 的 build 方法创建 Glide 对象，并且注册了回调，用于监听回调。
GlideBuilder 类：
public final class GlideBuilder {
...
@NonNull
Glide build(@NonNull Context context) {
if (sourceExecutor == null) {//创建网络加载线程池对象
sourceExecutor = GlideExecutor.newSourceExecutor();
}
if (diskCacheExecutor == null) {//创建磁盘加载线程池对象
diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();
}
if (animationExecutor == null) {//创建动画加载线程池对象
animationExecutor = GlideExecutor.newAnimationExecutor();
}
if (memorySizeCalculator == null) { //创建内存大小计算器
memorySizeCalculator = new MemorySizeCalculator.Builder(context).build();
}
if (connectivityMonitorFactory == null) {//创建默认的网络连接监听工厂
connectivityMonitorFactory = new DefaultConnectivityMonitorFactory();
}
if (bitmapPool == null) {
int size = memorySizeCalculator.getBitmapPoolSize();
if (size > 0) {//如果Bitmap池的大小大于0，采用LruBitmapPool
bitmapPool = new LruBitmapPool(size);
} else {
bitmapPool = new BitmapPoolAdapter();
}
}
if (arrayPool == null) {
//arrayPool是对数组池，主要用于图片解析时存储临时数据用
//根据数组池的大小创建LruArrayPool
arrayPool = new LruArrayPool(memorySizeCalculator.getArrayPoolSizeInBytes());
}
if (memoryCache == null) {//创建内存缓存实现类
memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
}
if (diskCacheFactory == null) {//创建磁盘缓存工厂类
diskCacheFactory = new InternalCacheDiskCacheFactory(context);
}
if (engine == null) { //创建图片加载引擎
engine =
new Engine(
memoryCache,
diskCacheFactory,
diskCacheExecutor,
sourceExecutor,
GlideExecutor.newUnlimitedSourceExecutor(),
animationExecutor,
isActiveResourceRetentionAllowed);
}
if (defaultRequestListeners == null) {
defaultRequestListeners = Collections.emptyList();
} else {
defaultRequestListeners = Collections.unmodifiableList(defaultRequestListeners);
}
//创建RequestManagerRetriever对象
RequestManagerRetriever requestManagerRetriever =
new RequestManagerRetriever(requestManagerFactory);
return new Glide(
context,
engine,
memoryCache,
bitmapPool,
arrayPool,
requestManagerRetriever,
connectivityMonitorFactory,
logLevel,
defaultRequestOptionsFactory,
defaultTransitionOptions,
defaultRequestListeners,
isLoggingRequestOriginsEnabled,
isImageDecoderEnabledForBitmaps);
}
}
GlideBuilder 默认创建了网络加载线程池、磁盘缓存加载线程池、动画线程池、Bitmap池（用于复用）、数组池（用于复用）、内存缓存、磁盘缓存、图片加载引擎等。然后创建Glide。
在这里 requestManagerRetriever 作为参数传递给 Glide 的构造方法中，在 with 方法中：
@NonNull
public static RequestManager with(@NonNull Activity activity) {
return getRetriever(activity).get(activity);
}getRetriever(activity) 获取到的就是 requestManagerRetriever ，接从 RequestManagerRetriever 中的 get 方法获取到 RequestManager 。
小结：
通过 GlideBuilder 创建 RequestManagerRetriever 对象，并当做参数传入 Glide 的构造方法中。GlideBuilder 默认创建了网络加载线程池、磁盘缓存加载线程池、动画线程池、Bitmap池（用于复用）、数组池（用于复用）、内存缓存、磁盘缓存、图片加载引擎等。采用单例模式的方式获取到 Glide 对象，并通过 RequestManagerRetriever 对象获取 RequestManager 对象。
3. load(url)方法
从上面 Glide.with() 方法最终获取到 RequestManager 对象，load 方法就是 RequestManager 中的方法。RequestManager 类图如下：
load 方法：
public RequestBuilder<Drawable> load(@Nullable Bitmap bitmap) {
return asDrawable().load(bitmap);
}
public RequestBuilder<Drawable> load(@Nullable Drawable drawable) {
return asDrawable().load(drawable);
}
public RequestBuilder<Drawable> load(@Nullable String string) {
return asDrawable().load(string);
}
public RequestBuilder<Drawable> load(@Nullable Uri uri) {
return asDrawable().load(uri);
}
public RequestBuilder<Drawable> load(@Nullable File file) {
return asDrawable().load(file);
}
public RequestBuilder<Drawable> load(@RawRes @DrawableRes @Nullable Integer resourceId)
{
return asDrawable().load(resourceId);
}
public RequestBuilder<Drawable> load(@Nullable URL url) {
return asDrawable().load(url);
}
public RequestBuilder<Drawable> load(@Nullable byte[] model) {
return asDrawable().load(model);
}
public RequestBuilder<Drawable> load(@Nullable Object model) {
return asDrawable().load(model);
}重载了多个的 load 方法，但是有个共同点都是调用 asDrawable() 方法：
public RequestBuilder<Drawable> asDrawable() {
return as(Drawable.class);
}
public <ResourceType> RequestBuilder<ResourceType> as(
@NonNull Class<ResourceType> resourceClass) {
return new RequestBuilder<>(glide, this, resourceClass, context);
}这里最终把 glide 和 context 作为参数创建了 RequestBuilder 对象，所以这里调用了 RequestBuilder 中的 load 方法，在文章开头传递了 String 类型的 URL 地址：
public RequestBuilder<TranscodeType> load(@Nullable String string) {
return loadGeneric(string);
}
@NonNull
private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
this.model = model;
isModelSet = true;
return this;
}这里只是将传递进来的 String 类型图片地址赋值给 model 变量。
小结：
load 方法中，通过 RequestManager 创建了 RequestBuilder 对象，并将图片地址信息等赋值给 RequestBuilder 中的 model 变量。
4. into(imageView) 方法
在上一步中最终返回的 RequestBuilder 对象，into 就是 RequestBuilder 中的方法：
public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
Util.assertMainThread();
Preconditions.checkNotNull(view);
BaseRequestOptions<?> requestOptions = this;
if (!requestOptions.isTransformationSet()
&& requestOptions.isTransformationAllowed()
&& view.getScaleType() != null) {
// Clone in this method so that if we use this RequestBuilder to load into a View and then
// into a different target, we don't retain the transformation applied based on the previous
// View's scale type.
switch (view.getScaleType()) {
case CENTER_CROP:
requestOptions = requestOptions.clone().optionalCenterCrop();
break;
case CENTER_INSIDE:
requestOptions = requestOptions.clone().optionalCenterInside();
break;
case FIT_CENTER:
case FIT_START:
case FIT_END:
requestOptions = requestOptions.clone().optionalFitCenter();
break;
case FIT_XY:
requestOptions = requestOptions.clone().optionalCenterInside();
break;
case CENTER:
case MATRIX:
default:
// Do nothing.
}
}
return into(
glideContext.buildImageViewTarget(view, transcodeClass),
/*targetListener=*/ null,
requestOptions,
Executors.mainThreadExecutor());
}先获取 ImageView 的缩放类型，通过 glideContext.buildImageViewTarget(view, transcodeClass) 创建一个 Target 对象，buildImageViewTarget 方法：
@NonNull
public <X> ViewTarget<ImageView, X> buildImageViewTarget(
@NonNull ImageView imageView, @NonNull Class<X> transcodeClass) {
return imageViewTargetFactory.buildTarget(imageView, transcodeClass);
}public class ImageViewTargetFactory {
@NonNull
@SuppressWarnings("unchecked")
public <Z> ViewTarget<ImageView, Z> buildTarget(
@NonNull ImageView view, @NonNull Class<Z> clazz) {
if (Bitmap.class.equals(clazz)) {
return (ViewTarget<ImageView, Z>) new BitmapImageViewTarget(view);
} else if (Drawable.class.isAssignableFrom(clazz)) {
return (ViewTarget<ImageView, Z>) new DrawableImageViewTarget(view);
} else {
throw new IllegalArgumentException(
"Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)");
}
}
}这边创建的是一个 DrawableImageViewTarget 对象。
继续看上面的 into 方法：
private <Y extends Target<TranscodeType>> Y into(
@NonNull Y target,
@Nullable RequestListener<TranscodeType> targetListener,
BaseRequestOptions<?> options,
Executor callbackExecutor) {
...
//创建一个Request对象，target 是前面创建的DrawableImageViewTarget对象
Request request = buildRequest(target, targetListener, options, callbackExecutor);
//拿到上一个请求
Request previous = target.getRequest();
//判断是否与上一个请求冲突，一般不用管
if (request.isEquivalentTo(previous)
&& !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
if (!Preconditions.checkNotNull(previous).isRunning()) {
previous.begin();
}
return target;
}
requestManager.clear(target);
target.setRequest(request);
requestManager.track(target, request);
return target;
}这里的 request 返回的是一个 SingleRequest 对象，最后调用的是 RequestManager 的 track(target, request) 方法。SingleRequest 创建的流程可以参考下面的流程图：
继续看 RequestManager 的 track 方法，传入 DrawableImageViewTarget target 和 SingleRequest request：
synchronized void track(@NonNull Target<?> target, @NonNull Request request) {
targetTracker.track(target);
requestTracker.runRequest(request);
}TargetTracker 类实现了接口 LifecycleListener 可以感应生命周期的变化：
public final class TargetTracker implements LifecycleListener {
private final Set<Target<?>> targets =
Collections.newSetFromMap(new WeakHashMap<Target<?>, Boolean>());
public void track(@NonNull Target<?> target) {
targets.add(target);
}
public void untrack(@NonNull Target<?> target) {
targets.remove(target);
}
@Override
public void onStart() {
for (Target<?> target : Util.getSnapshot(targets)) {
target.onStart();
}
}
@Override
public void onStop() {
for (Target<?> target : Util.getSnapshot(targets)) {
target.onStop();
}
}
@Override
public void onDestroy() {
for (Target<?> target : Util.getSnapshot(targets)) {
target.onDestroy();
}
}
@NonNull
public List<Target<?>> getAll() {
return Util.getSnapshot(targets);
}
public void clear() {
targets.clear();
}
}接着看 RequestTracker 的 runRequest 方法：
public void runRequest(@NonNull Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
request.clear();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Paused, delaying request");
}
pendingRequests.add(request);
}
}如果当前请求是停止的，则将请求删除，如果不是停止的，则调用 Request 的 begin 方法，上面得知 Request 是 SingleRequest 的 begin 方法：
public void begin() {
synchronized (requestLock) {
...
if (status == Status.RUNNING) {
throw new IllegalArgumentException("Cannot restart a running request");
}
if (status == Status.COMPLETE) {
onResourceReady(resource, DataSource.MEMORY_CACHE);
return;
}
status = Status.WAITING_FOR_SIZE;
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)
&& canNotifyStatusChanged()) {
target.onLoadStarted(getPlaceholderDrawable());
}
...
}
}当请求状态为进行中，则抛出异常，当请求的状态为完成，则调用方法 onResourceReady 从缓存中获取数据，等研究缓存机制的时候再来看这边的代码。
接着会调用 target.getSize(this) 根据 View 的宽高来计算出图片的宽高，最后回调的是 onSizeReady 方法：
public void onSizeReady(int width, int height) {
stateVerifier.throwIfRecycled();
synchronized (requestLock) {
...
status = Status.RUNNING;//切换状态为运行中
...
loadStatus =
engine.load(
glideContext,
model,
requestOptions.getSignature(),
this.width,
this.height,
requestOptions.getResourceClass(),
transcodeClass,
priority,
requestOptions.getDiskCacheStrategy(),
requestOptions.getTransformations(),
requestOptions.isTransformationRequired(),
requestOptions.isScaleOnlyOrNoTransform(),
requestOptions.getOptions(),
requestOptions.isMemoryCacheable(),
requestOptions.getUseUnlimitedSourceGeneratorsPool(),
requestOptions.getUseAnimationPool(),
requestOptions.getOnlyRetrieveFromCache(),
this,
callbackExecutor);
...
}
}调用了 Engine 的 load 方法，Engine 的类图如下所示：
load 方法：
public <R> LoadStatus load(
GlideContext glideContext,
Object model,
Key signature,
int width,
int height,
Class<?> resourceClass,
Class<R> transcodeClass,
Priority priority,
DiskCacheStrategy diskCacheStrategy,
Map<Class<?>, Transformation<?>> transformations,
boolean isTransformationRequired,
boolean isScaleOnlyOrNoTransform,
Options options,
boolean isMemoryCacheable,
boolean useUnlimitedSourceExecutorPool,
boolean useAnimationPool,
boolean onlyRetrieveFromCache,
ResourceCallback cb,
Executor callbackExecutor) {
long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;
EngineKey key =
keyFactory.buildKey(
model,
signature,
width,
height,
transformations,
resourceClass,
transcodeClass,
options);
EngineResource<?> memoryResource;
synchronized (this) {
memoryResource = loadFromMemory(key, isMemoryCacheable, startTime);
if (memoryResource == null) {
return waitForExistingOrStartNewJob(
glideContext,
model,
signature,
width,
height,
resourceClass,
transcodeClass,
priority,
diskCacheStrategy,
transformations,
isTransformationRequired,
isScaleOnlyOrNoTransform,
options,
isMemoryCacheable,
useUnlimitedSourceExecutorPool,
useAnimationPool,
onlyRetrieveFromCache,
cb,
callbackExecutor,
key,
startTime);
}
}
cb.onResourceReady(memoryResource, DataSource.MEMORY_CACHE);
return null;
}首先根据一系列条件计算出每张图片对应的 key，key 生成的因素有很多，比如宽高等；
接着调用 loadFromMemory 方法，先从缓存中获取，如果缓存中有，则直接返回 EngineResource 对象。等研究缓存机制的时候再来看这边的代码。
如果从缓存中获取不到对应的资源，则调用 waitForExistingOrStartNewJob 方法：
private <R> LoadStatus waitForExistingOrStartNewJob(
GlideContext glideContext,
Object model,
Key signature,
int width,
int height,
Class<?> resourceClass,
Class<R> transcodeClass,
Priority priority,
DiskCacheStrategy diskCacheStrategy,
Map<Class<?>, Transformation<?>> transformations,
boolean isTransformationRequired,
boolean isScaleOnlyOrNoTransform,
Options options,
boolean isMemoryCacheable,
boolean useUnlimitedSourceExecutorPool,
boolean useAnimationPool,
boolean onlyRetrieveFromCache,
ResourceCallback cb,
Executor callbackExecutor,
EngineKey key,
long startTime) {
EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache);
if (current != null) {
current.addCallback(cb, callbackExecutor);
if (VERBOSE_IS_LOGGABLE) {
logWithTimeAndKey("Added to existing load", startTime, key);
}
return new LoadStatus(cb, current);
}
EngineJob<R> engineJob =
engineJobFactory.build(
key,
isMemoryCacheable,
useUnlimitedSourceExecutorPool,
useAnimationPool,
onlyRetrieveFromCache);
DecodeJob<R> decodeJob =
decodeJobFactory.build(
glideContext,
model,
key,
signature,
width,
height,
resourceClass,
transcodeClass,
priority,
diskCacheStrategy,
transformations,
isTransformationRequired,
isScaleOnlyOrNoTransform,
onlyRetrieveFromCache,
options,
engineJob);
jobs.put(key, engineJob);
engineJob.addCallback(cb, callbackExecutor);
engineJob.start(decodeJob);
if (VERBOSE_IS_LOGGABLE) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}根据 key 判断是否正在执行，如果正在执行则调用回调方法。如果不存在，则构建新的请求任务，并且添加回调。还创建了 DecodeJob 用于图片解码。
最后调用了 EngineJob 的 start 方法，并将 decodeJob 作为参数传进去：
public synchronized void start(DecodeJob<R> decodeJob) {
this.decodeJob = decodeJob;
GlideExecutor executor =
decodeJob.willDecodeFromCache() ? diskCacheExecutor : getActiveSourceExecutor();
executor.execute(decodeJob);
}start 中只是调用了 GlideExecuto r的 execute 方法：
@Override
public void execute(@NonNull Runnable command) {
delegate.execute(command);
}GlideExecutor 开启了一个新线程去加载资源，就从主线程转到子线程，进入到 DecodeJob 类中找到 run 方法：
public void run() {
GlideTrace.beginSectionFormat("DecodeJob#run(model=%s)", model);
DataFetcher<?> localFetcher = currentFetcher;
try {
if (isCancelled) {
notifyFailed();
return;
}
runWrapped();
} catch (CallbackException e) {
throw e;
} catch (Throwable t) {
if (stage != Stage.ENCODE) {
throwables.add(t);
notifyFailed();
}
if (!isCancelled) {
throw t;
}
throw t;
} finally {
if (localFetcher != null) {
localFetcher.cleanup();
}
GlideTrace.endSection();
}
}主要是执行了 runWrapped 方法：
private void runWrapped() {
switch (runReason) {
case INITIALIZE:
stage = getNextStage(Stage.INITIALIZE);
currentGenerator = getNextGenerator();
runGenerators();
break;
case SWITCH_TO_SOURCE_SERVICE:
runGenerators();
break;
case DECODE_DATA:
decodeFromRetrievedData();
break;
default:
throw new IllegalStateException("Unrecognized run reason: " + runReason);
}
}最开始执行的是 INITIALIZE 的case，getNextGenerator 返回的是 SourceGenerator，接着走的是 runGenerators 方法：
private void runGenerators() {
currentThread = Thread.currentThread();
startFetchTime = LogTime.getLogTime();
boolean isStarted = false;
while (!isCancelled
&& currentGenerator != null
&& !(isStarted = currentGenerator.startNext())) {
stage = getNextStage(stage);
currentGenerator = getNextGenerator();
if (stage == Stage.SOURCE) {
reschedule();
return;
}
}
if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
notifyFailed();
}
}在 while 循环中调用了 currentGenerator 的 startNext() 方法，也就是 SourceGenerator 的 startNext() 方法：
@Override
public boolean startNext() {
if (dataToCache != null) {
Object data = dataToCache;
dataToCache = null;
cacheData(data);
}
if (sourceCacheGenerator != null && sourceCacheGenerator.startNext()) {
return true;
}
sourceCacheGenerator = null;
loadData = null;
boolean started = false;
while (!started && hasNextModelLoader()) {
//获取到 ModelLoader
loadData = helper.getLoadData().get(loadDataListIndex++);
if (loadData != null
&& (helper.getDiskCacheStrategy().isDataCacheable(loadData.fetcher.getDataSource())
|| helper.hasLoadPath(loadData.fetcher.getDataClass()))) {
started = true;
startNextLoad(loadData);
}
}
return started;
}接着看 startNextLoad：
private void startNextLoad(final LoadData<?> toStart) {
loadData.fetcher.loadData(
helper.getPriority(),
new DataCallback<Object>() {
@Override
public void onDataReady(@Nullable Object data) {
if (isCurrentRequest(toStart)) {
onDataReadyInternal(toStart, data);
}
}
@Override
public void onLoadFailed(@NonNull Exception e) {
if (isCurrentRequest(toStart)) {
onLoadFailedInternal(toStart, e);
}
}
});
}这里 loadData.fetch 是 HttpUrlFetcher，loadData 方法：
public void loadData(
@NonNull Priority priority, @NonNull DataCallback<? super InputStream> callback) {
long startTime = LogTime.getLogTime();
try {
InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());
callback.onDataReady(result);
} catch (IOException e) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "Failed to load data for url", e);
}
callback.onLoadFailed(e);
} finally {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime));
}
}
}通过 loadDataWithRedirects 方法返回一个输入流：
private InputStream loadDataWithRedirects(
URL url, int redirects, URL lastUrl, Map<String, String> headers) throws IOException {
if (redirects >= MAXIMUM_REDIRECTS) {
throw new HttpException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
} else {
// Comparing the URLs using .equals performs additional network I/O and is generally broken.
// See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.
try {
if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
throw new HttpException("In re-direct loop");
}
} catch (URISyntaxException e) {
// Do nothing, this is best effort.
}
}
urlConnection = connectionFactory.build(url);
for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
}
urlConnection.setConnectTimeout(timeout);
urlConnection.setReadTimeout(timeout);
urlConnection.setUseCaches(false);
urlConnection.setDoInput(true);
// Stop the urlConnection instance of HttpUrlConnection from following redirects so that
// redirects will be handled by recursive calls to this method, loadDataWithRedirects.
urlConnection.setInstanceFollowRedirects(false);
// Connect explicitly to avoid errors in decoders if connection fails.
urlConnection.connect();
// Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
stream = urlConnection.getInputStream();
if (isCancelled) {
return null;
}
final int statusCode = urlConnection.getResponseCode();
if (isHttpOk(statusCode)) {
return getStreamForSuccessfulRequest(urlConnection);
} else if (isHttpRedirect(statusCode)) {
String redirectUrlString = urlConnection.getHeaderField("Location");
if (TextUtils.isEmpty(redirectUrlString)) {
throw new HttpException("Received empty or null redirect url");
}
URL redirectUrl = new URL(url, redirectUrlString);
// Closing the stream specifically is required to avoid leaking ResponseBodys in addition
// to disconnecting the url connection below. See #2352.
cleanup();
return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
} else if (statusCode == INVALID_STATUS_CODE) {
throw new HttpException(statusCode);
} else {
throw new HttpException(urlConnection.getResponseMessage(), statusCode);
}
}这是熟悉的 Http 请求了，用的是 HttpURLConnection 为 Glide 底层完成网络请求的。通过 onDataReady 回调到 SourceGenerator 的 startNextLoad 函数中，接着调用 onDataReadyInternal 方法：
void onDataReadyInternal(LoadData<?> loadData, Object data) {
DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();
if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {
dataToCache = data;
// We might be being called back on someone else's thread. Before doing anything, we should
// reschedule to get back onto Glide's thread.
cb.reschedule();
} else {
cb.onDataFetcherReady(
loadData.sourceKey,
data,
loadData.fetcher,
loadData.fetcher.getDataSource(),
originalKey);
}
}如果没有配置缓存，则继续执行回调 onDataFetcherReady，这里的 cb 是 FetcherReadyCallback，它的实现类是 DecodeJob：
public void onDataFetcherReady(
Key sourceKey, Object data, DataFetcher<?> fetcher, DataSource dataSource, Key attemptedKey) {
this.currentSourceKey = sourceKey;
this.currentData = data;
this.currentFetcher = fetcher;
this.currentDataSource = dataSource;
this.currentAttemptingKey = attemptedKey;
if (Thread.currentThread() != currentThread) {
runReason = RunReason.DECODE_DATA;
callback.reschedule(this);
} else {
GlideTrace.beginSection("DecodeJob.decodeFromRetrievedData");
try {
decodeFromRetrievedData();
} finally {
GlideTrace.endSection();
}
}
}接着调用 decodeFromRetrievedData 解析返回的数据：
private void decodeFromRetrievedData() {
Resource<R> resource = null;
try {
resource = decodeFromData(currentFetcher, currentData, currentDataSource);
} catch (GlideException e) {
e.setLoggingDetails(currentAttemptingKey, currentDataSource);
throwables.add(e);
}
if (resource != null) {
notifyEncodeAndRelease(resource, currentDataSource);
} else {
runGenerators();
}
}这里返回 Resource 对象，紧接着调用 notifyEncodeAndRelease 方法：
private void notifyEncodeAndRelease(Resource<R> resource, DataSource dataSource) {
...
notifyComplete(result, dataSource);
...
}notifyEncodeAndRelease 方法：
private void notifyComplete(Resource<R> resource, DataSource dataSource) {
setNotifiedOrThrow();
callback.onResourceReady(resource, dataSource);
}这里的 callback 的实现类是 EngineJob ，这里再次回到 EngineJob 中的 onResourceReady 方法：
public void onResourceReady(Resource<R> resource, DataSource dataSource) {
synchronized (this) {
this.resource = resource;
this.dataSource = dataSource;
}
notifyCallbacksOfResult();
}调用了 notifyCallbacksOfResult 方法：
void notifyCallbacksOfResult() {
...
synchronized (this) {
...
engineResource = engineResourceFactory.build(resource, isCacheable, key, resourceListener);
hasResource = true;
copy = cbs.copy();
incrementPendingCallbacks(copy.size() + 1);
localKey = key;
localResource = engineResource;
}
engineJobListener.onEngineJobComplete(this, localKey, localResource);
for (final ResourceCallbackAndExecutor entry : copy) {
entry.executor.execute(new CallResourceReady(entry.cb));
}
decrementPendingCallbacks();
}调用 onEngineJobComplete 通知 Engine 任务完成，接着调用 entry.executor.execute(new CallResourceReady(entry.cb))，这里核心方法是 CallResourceReady：
private class CallResourceReady implements Runnable {
private final ResourceCallback cb;
CallResourceReady(ResourceCallback cb) {
this.cb = cb;
}
@Override
public void run() {
// Make sure we always acquire the request lock, then the EngineJob lock to avoid deadlock
// (b/136032534).
synchronized (cb.getLock()) {
synchronized (EngineJob.this) {
if (cbs.contains(cb)) {
// Acquire for this particular callback.
engineResource.acquire();
callCallbackOnResourceReady(cb);
removeCallback(cb);
}
decrementPendingCallbacks();
}
}
}
}CallResourceReady 实现 Runnable 接口，当 entry.executor.execute 线程池执行的时候就会调用 run 方法，接着继续看方法 callCallbackOnResourceReady：
void callCallbackOnResourceReady(ResourceCallback cb) {
try {
cb.onResourceReady(engineResource, dataSource);
} catch (Throwable t) {
throw new CallbackException(t);
}
}这里的回调返回给 SingleRequest 中的 onResourceReady 方法：
@Override
public void onResourceReady(Resource<?> resource, DataSource dataSource) {
stateVerifier.throwIfRecycled();
Resource<?> toRelease = null;
try {
synchronized (requestLock) {
loadStatus = null;
if (resource == null) {
...
onLoadFailed(exception);
return;
}
Object received = resource.get();
if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
...
onLoadFailed(exception);
return;
}
...
onResourceReady((Resource<R>) resource, (R) received, dataSource);
}
} finally {
if (toRelease != null) {
engine.release(toRelease);
}
}
}如果返回的 resource 为空，则调用 onLoadFailed，接着的流程为 onResourceReady：
private void onResourceReady(Resource<R> resource, R result, DataSource dataSource) {
// We must call isFirstReadyResource before setting status.
boolean isFirstResource = isFirstReadyResource();
status = Status.COMPLETE;
this.resource = resource;
...
isCallingCallbacks = true;
try {
...
if (!anyListenerHandledUpdatingTarget) {
Transition<? super R> animation = animationFactory.build(dataSource, isFirstResource);
target.onResourceReady(result, animation);
}
} finally {
isCallingCallbacks = false;
}
notifyLoadSuccess();
}核心代码是 target.onResourceReady(result, animation) ，这里的 target 是 DrawableImageViewTarget，而 DrawableImageViewTarget 的父类是 ImageViewTarget，这里调用的是 ImageViewTarget 的 onResourceReady 方法：
@Override
public void onResourceReady(@NonNull Z resource, @Nullable Transition<? super Z> transition) {
if (transition == null || !transition.transition(resource, this)) {
setResourceInternal(resource);
} else {
maybeUpdateAnimatable(resource);
}
}调用了 setResourceInternal 方法：
private void setResourceInternal(@Nullable Z resource) {
// Order matters here. Set the resource first to make sure that the Drawable has a valid and
// non-null Callback before starting it.
setResource(resource);
maybeUpdateAnimatable(resource);
}setResource 的实现在子类 DrawableImageViewTarget 中：
@Override
protected void setResource(@Nullable Drawable resource) {
view.setImageDrawable(resource);
}在这里图片就真正的显示出来了。
以下是 into 阶段的相关流程图。
5. 总结
在 with 方法中通过 GlideBuilder 创建 RequestManagerRetriever 对象，并当做参数传入 Glide 的构造方法中。GlideBuilder 默认创建了网络加载线程池、磁盘缓存加载线程池、动画线程池、Bitmap池（用于复用）、数组池（用于复用）、内存缓存、磁盘缓存、图片加载引擎等。采用单例模式的方式获取到 Glide 对象，并通过 RequestManagerRetriever 对象获取 RequestManager 对象。RequestManager 创建了 RequestBuilder 对象，并将图片地址信息等赋值给 RequestBuilder 中的 model 变量。RequestManager 中的 TargetTracker 实现了 LifecycleListener 可以感应生命周期的变化。RequestBuilder 创建 了 SingleRequest 对象，SingleRequest 中创建了 Engine，在 Engine 中先检查是否存在活动资源缓存，以及内存缓存，如果没有缓存，则创建 EngineJob 来创建一个新的任务。在执行请求任务之前先检查磁盘缓存，如果不存在，则执行请求，默认的采用HttpURLConnection 为 Glide 底层完成网络请求的。最后通过回调将加载后的资源在 DrawableImageViewTarget 中将图片显示出来。
来源：oschina
链接：https://my.oschina.net/u/4381723/blog/4355661